1. Field of the Invention
The present invention relates to an image scanning apparatus and method for scanning an image on a reading medium such as a sheet of paper, and a mirror drive method and mechanism for rotating a reflecting mirror.
2. Description of the Related Art
Conventionally, a flat bed type image scanner has been widely used for reading/scanning an image on a surface of a reading medium such as a manuscript. This flat bed type image scanner normally has a contact glass arranged in a horizontal direction as medium placement means. Below this contact glass, there is provided an image scanning mechanism.
This image scanning mechanism includes, for example, an image sensor having a number of photo elements arranged in a main scanning direction and a support body for supporting the image sensor at a position parallel to the contact glass in such a manner that the image sensor can be moved in a sub scanning direction. In this flat bed type image scanner, the image on a reading medium placed on the contact glass can be scanned as a number of main scanning lines continuous in the sub scanning direction.
It should be noted that as such a flat bed type image scanner, there is a model in which the image sensor is fixed and a pair of scanning mirrors is moved in the sub scanning direction for scanning an image on a reading medium. Moreover, there is an image scanning apparatus in which the image sensor is fixed and the contact glass is moved in the sub scanning direction. Furthermore, there is an image scanning apparatus in which the image sensor is fixed and a manuscript as the reading medium is moved in the sub scanning direction by a sheet feed mechanism.
In the aforementioned image scanning apparatuses using the contact glass, the reading medium is placed with its image downward on the contact glass and a user cannot observe the image which is being scanned.
Moreover, in the model having a fixed image sensor, a reading medium is placed with its image upward on a sheet feeding mechanism and a user can observe the image which is being scanned. However, a jam may be caused in the sheet feeding mechanism when comparatively thin sheets of paper are set on the sheet feeding mechanism.
These problems are solved by image scanning apparatuses disclosed in Japanese Patent Applications No. 6-13307 and No. 6-294160. These image scanning apparatuses includes a table as medium arrangement means on which a reading medium is placed, and above this table, there is provided a scanner head supported by a support arm.
The scanner head has an image sensor, an image formation optical system, and a reflecting mirror which are built in the scanner head. The reflecting mirror is rotatably supported on a rotary shaft which is arranged in parallel to the main scanning direction. The reflecting mirror reflects an image light, i.e., a reflected light or transmitting light from the reading medium, into a predetermined direction which vertically intersects the main scanning direction.
This image sensor has one-dimensional structure consisting of a number of photo elements arranged in the main scan direction so as to read a main scan line consisting of a number of dots. Furthermore, reflecting mirror is rotated so that the main scan line reading position is shifted in the sub scan direction. Thus, a two-dimensional image is read/scanned by the number of main scan lines continuous in the sub scan direction.
In the image scanning apparatus having the aforementioned configuration, a reading medium is placed with its image facing upward on the table. That is, a user can observe the image while it is being scanned. Moreover, there is no need of feeding a reading medium by a feeding mechanism. Accordingly, there is no problem in reading an image on a very thin sheet of paper or on a page of a thick book.
In the aforementioned image scanning apparatus, there is a problem that during the sub scanning on the scan medium carried out by the rotation of the reflecting mirror, the sub scan speed of the scan position on the scan medium is fluctuated if the reflecting mirror is rotated at a constant angular velocity.
That is, at a center position where an image is scanned at a right angle, the sub scan linear velocity becomes smallest, and the linear velocity is increased as the scan position is shifted from the center to the end portion of the manuscript. If the image sensor reads main scan lines at a constant interval in this state, the image data obtained is such that a center potion is expanded and the both end portions are contracted.
For example, it is possible to electronically control the image sensor scan interval corresponding to the rotation angle of the reflecting mirror or to correct by way of data processing the image data scanned. However, these require a complicated processing operation. In the former case, the light reception amount of the image sensor is varied and in the latter case, the resolution of the image data is finally lowered.
In order to solve the aforementioned problems, conventionally, the angular velocity of the reflecting mirror is changed in a non-linear way so that the scan position on the scan medium is moved in the sub scan direction is a constant linear velocity. Thus, for controlling the angular velocity of the reflecting mirror, a voice coil motor has been used as a drive source for servo control of the reflecting mirror rotation. However, this complicates the circuit configuration, reducing the productivity and disabling to reduce the apparatus size.